(i) Field of the Invention
This invention relates to a process for the preparation of phosphoric diesters of high purity and more particularly, to a process for preparing phosphoric diesters of high purity by selectively hydrolyzing phosphoric monoesters in phosphoric ester mixtures comprising phosphoric monoesters and phosphoric diesters, and removing the resulting hydrolyzates from the mixtures.
(ii) Description of the Prior Art
Phosphoric esters of organic hydroxyl compounds are utilized as a detergent, emulsifier, antistatic agent, oil for fibers, and the like. In these applications, phosphoric diesters are frequently employed as mixtures with phosphoric monoesters, phosphoric triesters or condensed phosphoric esters.
On the other hand, highly pure phosphoric diester such as, for example, di-2-ethylhexyl phosphate, is an important compound which has been widely used as a liquid ion exchanger (solvent for metal extraction) in the fields of refining of uranium ores, recovery of uranium from phosphoric acid obtained by a wet process, refining of nickel, cobalt, rare earth elements, and the like, or recovery of valuable metals from waste liquors discharged from surface treatments of metallic materials.
Phosphoric esters of organic hydroxyl compounds are industrially produced by reaction of phosphorylating agents such as phosphorus oxychloride, phosphorus pentaoxide, polyphosphoric acid, and the like with organic hydroxyl compounds. For the production of phosphoric diesters, it is usual to use phosphorus oxychloride. When phosphorus pentaoxide is used as a phosphorylating agent, a so-called sesquiphosphate (comprising a mixture of almost equimolar amounts of phosphoric diester and phosphoric monoester along with condensed phosphoric esters) is obtained. With polyphosphoric acids, phosphoric diesters are not substantially produced.
In general, the reaction between organic hydroxyl compounds and phosphorus oxychloride for the production of phosphoric diesters is complicated. Products obtained by ordinary production processes usually contain impurities such as phosphoric monoesters and phosphoric triesters, and the purity of phosphoric diester is at most as low as 70-80%.
In order to obtain highly pure phosphoric diesters, purification is necessary. This is ordinarily effected by utilizing a difference in the physical property such as, for example, solubility between a phosphoric diester and impurities such as a phosphoric monoester. However, for example in the purification by extraction of di-2-ethylhexyl phosphate, phase separation becomes very poor because only a small amount of mono-2-ethylhexyl phosphate is present as an impurity. This will produce a great trouble for the purification on an industrial scale.